Regarding electricity consumption of slim display devices, various efforts to reduce electricity consumption of a backlight, such as a use of LEDs (Light Emitting Diodes) in a backlight (hereinafter, also referred to as B/L), have been made in recent years. However, the ratio of electricity consumption of a backlight to the total electricity consumption of a display device is now still great. In view of that, a reduction of the electricity consumption by using a technology to control the luminance of a backlight according to image signals, is now being made. Further, there has been proposed another technology to furthermore reduce electricity consumption of a backlight. That is, a technology of a RGBW display device which employs W (white) pixels in addition to R (Red), G (Green) and B (Blue) pixels so as to enhance the luminance, is combined with a technology to control the luminance of a backlight. The extent of enhanced luminance in the RGBW display device is used for reducing the luminance of the backlight, so as to furthermore reduce the electricity consumption of the backlight.
For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2007-10753 discloses one of such technologies to reduce the electricity consumption of a backlight. That is, the disclosed technology uses an image data conversion circuit illustrated in FIG. 12 to drive a backlight, where the drive process includes a control of luminance of the backlight. In the drive process, a gradation conversion is applied onto image data so as to make the maximum value of data assigned to each color pixels the same as each other, without maximizing the value of data assigned to white pixels. Thereby, the gradation-extension rate given after the gradation conversion is increased.
In concrete terms, image data inputted to the image data conversion circuit is first converted into RGBW data. In the image data conversion circuit, there are a four-color conversion circuit A for converting RGB data into RGBW data without chromatic and luminance changes, and a four-color conversion circuit B for converting RGB data into RGBW data with chromatic and luminance changes. The input image data is inputted to both the conversion circuits. On the basis of a level detection signal, either one of the RGBW data respectively outputted from the four-color conversion circuits A and B is selected by a selector. That is, if the data is regarded as that of the white peak region, the signal from the conversion circuit B is selected, and if the data is equal to or lower than normal 100% white, the signal from the conversion circuit A is selected. The RGBW data outputted from the selector is temporarily retained in a memory for a certain retention period. On the other hand, a maximum data value register retains the maximum values of the respective color data outputted during the retention period. After display data corresponding to one screen is retained in the memory, and the maximum data value for each color within the screen is set in the maximum data value register, a backlight luminance control circuit calculates the backlight light-emission quantity on the basis of the maximum data value for each color, and controls the light emission quantity of the backlight at the time of displaying the next screen. On the other hand, a backlight luminance compensation data conversion circuit sequentially reads display data in the memory, and after performing data conversion on the basis of the backlight light-emission quantity signal inputted from the backlight luminance control circuit so as to compensate for the backlight luminance, outputs the resultant data as the display data for the next screen.
Further, JP-A No. 2012-27405 discloses a technology to correct color shift of W pixels caused in a RGBW display device. Since there are provided no color filters for W pixels, W pixels cannot transmit light having selected wavelengths, which can cause a shift of the peak of the spectrum of the white light coming from the W pixels toward a short wavelength direction, depending on the gradation value. Therefore, the color tone of the white created by the W pixels can be different from that of white created by R, G and B pixels. In view of that, the disclosed technology employs an output signal generating section illustrated in FIG. 13, and conducts color conversion processing with the section after processing of RGBW signals, to correct the color shift of the W pixels by using all of the RGBW pixels.
The output signal generating section includes a backlight level calculating section, an LCD level calculating section, a chromaticity point adjusting section, and an RGB/RGBW converting section. The output signal generating section carries out a predetermined signal processing based on a video signal D1 (D1r, D1g, and D1b). By this, a lighting signal BL1 which shows luminance level (lighting level) in the backlight, and a video signal D4 (pixel signal D4r for R, pixel signal D4g for G and pixel signal D4b for B, and pixel signal D4w for W) or an output video signal are generated.
Further, JP-A No. 2009-086054 discloses the technology to lower the luminance of pixels by applying luminance lowering processing or chroma lowering processing onto signals of pixels each having a high gradation value, in a RGBW display device illustrated in FIG. 14, in order to increase the effect of the reduction of electricity consumption. This document shows the following formulas as a way to calculate the W value, and discloses that the effect of the reduction of electricity is increased by increasing the value to be distributed to the W sub-pixels.W=max(R,G,B)/2, for max(R,G,B)/2≧min(R,G,B)W=min(R,G,B), for max(R,G,B)/2<min(R,G,B)
The document further discloses that, in order to further provide the effect of the reduction of electricity consumption, luminance lowering processing and chroma lowering processing are also applied onto signals of pixels each having high gradation value, to reduce the electricity consumed by the backlight.
The RGBW display device illustrated in FIG. 14 includes a liquid crystal panel containing pixels each divided into four sub-pixels, a red (R), a green (G), a blue (B), and a white (W) sub-pixel, and a white backlight which emits light with controllable emission luminance. The RGBW display device further includes a luminance lowering section, an output signal generating section, a liquid crystal panel control section and a backlight control section. The luminance lowering section performs luminance lowering processing on high luminance pixel data of pixel data contained in input RGB signals representing an input image to transform the input RGB signals to luminance-lowered RGB signal. The output signal generating section generates transmittance signals for individual R, G, B, W sub-pixels in the pixels in the liquid crystal panel from the luminance-lowered RGB signals and also calculates a backlight value for the white backlight from the luminance-lowered RGB signals. The liquid crystal panel control section outputs panel control signals and controls driving of the liquid crystal panel according to the transmittance signals generated in the output signal generating section. The backlight control section outputs backlight control signals and controls the emission luminance of the backlight according to the backlight value calculated in the output signal generating section.
JP-A No. 2010-049011 discloses the following technology. In order to increase a reduction of electricity consumption of a RGBW display device illustrated in FIG. 15, the display device conducts luminance lowering processing and chroma lowering processing for signals of pixels each having a high gradation value, to lower the luminance of pixels. In this case, applying the luminance lowering processing and the chroma lowering processing onto signals of the pixels simply, can results in the situation that some pixels originally having different gradation values have the same resulting gradation value, which can causes white saturation (flattened gradation). In view of that, the display device conducts gradation correction by using a LUT (lookup table) before conducting the luminance lowering processing and the chroma lowering processing, and creates a conversion table so as to prevent the resulting data for pixels originally having different gradation values from having the same value, which avoids the flattening of gradation. The RGBW display device includes a liquid crystal panel containing pixels each divided into four sub-pixels, a red (R), a green (G), a blue (B), and a white (W) sub-pixel, and a white backlight which emits light with controllable emission luminance. The RGBW display device further includes a gradation correcting section, a chroma and luminance lowering section, a γ correction section, an output signal generating section, a liquid crystal panel control section and a backlight control section. The gradation correcting section reduces the signal value of an input RGB signal and converts the value into a gradation-corrected RGB signal. The chroma and luminance lowering section lowers the chroma and luminance of the gradation-corrected RGB signal and converts the gradation-corrected RGB signal into a chroma-and-luminance-lowered RGB signal. The γ correction section applies γ correction to the chroma-and-luminance-lowered RGB signal and converts it into a γ-corrected RGB signal. The output signal generating section generates a transmittance signal of a sub-pixel of each of R, G, B and W in each pixel of the liquid crystal panel from the γ-corrected RGB signal, and calculates a backlight value. The liquid crystal panel control section outputs a panel control signal and controls driving of the liquid crystal panel according to the transmittance signal generated in the output signal generating section. The backlight control section outputs a backlight control signal and controls the emission luminance of the backlight according to the backlight value calculated in the output signal generating section.
As described above, the realization of the luminance reduction of a backlight also needs a luminance control using a gradation conversion. However, in some display panels, because of their characteristics, the luminance control using the gradation conversion can cause a sense of strangeness about quality of a displayed image, for example, a conspicuous change in color tone of an area displayed in white due to the luminance control using a gradation conversion, which is a problem. It means that it is important to provide a control circuit for reducing the electricity consumption of a backlight with minimizing the sense of strangeness about the image quality.
Hereinafter, the change in color tone of the area displayed in white, coming from a gradation conversion, will be considered. For the consideration, there are provided an instance that an 8 bit-input display device displays a solid-color screen (rastered screen) in which the solid color has a gradation value of 255, namely, an all-white screen; and another instance that the 8 bit-input display device displays a window in red (R) having a gradation value of 255, as a primary color, in the above solid-color screen. In these instances, the maximum gradation value of the display device is assumed to be 255.
In the former instance that the display device displays the all-white screen, since there are no high-chroma pixels in the screen, all the W pixels over the screen are operated to light up fully (where the gradation value is 255). When the ratio of the luminance component of white created by a W pixel to the luminance component of white created by corresponding R, G and B pixels is 1 to 1, the total luminance of all the R, G, B and W pixels becomes twice as the luminance of R, G and B pixels. It enables the luminance of the backlight to be reduced to 50% of the base luminance, where the luminance of white color created by the R, G and B pixels in the RGBW display panel is used as the base luminance in the instances.
On the other hand, in the latter instance that the display device displays a window in red as primary color having the gradation value of 255 (R=255, G=0, B=0) in the all-white screen, since the red primary color of the gradation value of 255 is the highest in chroma all over the screen, the luminance of the backlight is hardly reduced in total. In the white area of the screen, the lighting level of each pixel can be defined individually. Therefore, the lighting level of each W pixel in the white area is set to be maximum, and the total luminance of the pixels in the white area becomes twice as the base luminance, which needs such excessively-high luminance of the pixels in the white area (R=255, G=255, B=255, W=255) to be reduced by 50%. By reducing the gradation value from 255 to 186, the luminance of the pixels in the white area can be reduced by 50%. Accordingly, applying a gradation conversion to R=186, G=186, B=186, W=186 onto the pixels results in the luminance reduced by 50%.
If a display panel having a characteristic that the chromaticity value when the display panel displays white of the gradation value of 186 is different from that when the display panel displays white of the gradation value of 255, is used in this instance, users can perceive a chromaticity difference of the displayed white color, which occurs due to a gradation conversion, and it results in a sense of strangeness about image quality. Concretely, in a instance that the display panel has a gradation-chromaticity characteristic that, as the gradation value changes from 255 to a smaller value, the chromaticity value represented by x, y coordinates of a xy chromaticity diagram changes to a smaller value, the displayed white color becomes bluish white as the gradation value decreases. In other words, the color of the white area, which has been displayed in pure white, changes to bluish white due to the red window displayed in the white screen, which causes a sense of strangeness about quality of the displayed image.
The present invention seeks to solve the problem.